The Smart Image Resizer from Shifting Pixels is an elegant, yet powerful way to resize and crop any image on a website without actually modifying the actual image or tweaking code. Images are uploaded once at a high resolution and then they can be reused in any format just by adjusting a query string - a big time saver.
Smart Image Resizer works with JPEGs, GIFs, and PNG files. It can sharpen images and crop based on height/width ratios, color fill transparent GIFs and PNGs and features built-in caching to keep image variations for the sake of performance. You'll need to be running PHP 5.1+ and the GD library should be installed.